Pressure Loss Is Explored in Vanishing of Jetliner

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Australian authorities have shifted the search for Malaysia Airlines Flight 370 further south in the Indian Ocean and said they believed that the plane was on autopilot.CreditCreditAlan Porritt/European Pressphoto Agency

CANBERRA, Australia — Nearly four months after Malaysia Airlines Flight 370 vanished into the night, Australian transportation officials said in a report on Thursday that the plane kept flying until it ran out of fuel, most likely because the cockpit crew had become unresponsive, perhaps because of oxygen deprivation.

The plane appears to have flown in a straight line south across the Indian Ocean, controlled entirely by the autopilot, Australian officials said. But they avoided offering hypotheses for why the plane had reached the northern end of the Indonesian island of Sumatra and had turned south in the first place, when it was supposed to travel the night of March 8 from Kuala Lumpur, the Malaysian capital, to Beijing.

Malaysian radar records show that the Boeing 777-200 did a U-turn over the Gulf of Thailand instead, then banked right across the Malaysian Peninsula and then banked right again to reach the northern tip of Sumatra.

Evidence of an unresponsive crew as the plane flew south for more than five hours includes the loss of radio communications, a long period with no maneuvering of the aircraft, a steady cruise altitude and eventual fuel exhaustion and descent, the report said. It added that this could have been caused by hypoxia, or oxygen deprivation, among the cockpit crew.

“Given these observations, the final stages of the unresponsive crew/hypoxia event type appeared to best fit the available evidence for the final period of MH370’s flight when it was heading in a generally southerly direction,” the document said.

Hypoxia occurs when a plane loses air pressure and the pilots, lacking adequate oxygen, become confused and incapable of performing even basic manual tasks, though they continue to feel confident in their own abilities.

Pilots are trained to put on oxygen masks immediately if an aircraft suffers depressurization; their masks have only an hour’s air supply, however. The plane, with 239 people aboard, made its turn south toward the Indian Ocean about an hour after it stopped responding to air traffic controllers. It is believed to have crashed somewhere in the southern Indian Ocean off the western coast of Australia. Australian officials have been coordinating the search.

Passengers’ masks have only a few minutes of oxygen, based on the theory that the pilot of a troubled plane will quickly descend to an altitude at which there is little need for a supplemental air supply.

The report added that the theory of an unresponsive crew possibly suffering from hypoxia was an operating assumption for the search and was not meant to infringe on Malaysia’s authority as the government responsible for conclusively identifying a cause for the loss of the plane.

There is no consensus among investigators, even within the Australian government, on the hypoxia or unresponsive-crew theory. Angus Houston, the retired head of the Australian military who is overseeing the country’s search, said in a telephone interview this month that he assumed that the flight had been on autopilot even if a conscious pilot had been at the controls. That is because a Boeing 777 is a very difficult plane to fly manually.

Other officials, who insisted on anonymity because of the diplomatic sensitivity of the issue with Malaysia and China — most of the flight’s passengers were Chinese — said some investigators still leaned toward the possibility that one of the pilots deliberately flew the plane to the southern Indian Ocean in a suicide mission that also killed everyone else aboard.

Advocates of the hypoxia theory argue that pilot suicide cases tend to involve pilots who crashed their planes suddenly, not after hours of flight. A clinical psychologist advising the investigation has been very skeptical of the suicide theory, saying it would be highly unusual for a suicidal person to proceed with such a deadly plan over many hours, investigators said.

Depressurization of an aircraft can occur from mechanical failure, an attempted hijacking or many other causes. If a plane undergoes gradual depressurization, pilots do not necessarily notice that they are losing oxygen, and with it, their mental clarity. Masks are supposed to deploy automatically with a loss of air pressure, but they need to be fitted properly for a full flow of oxygen.

At a news conference here on Thursday afternoon, Martin Dolan, the chief commissioner of the Australian Transport Safety Bureau, said someone on the plane had put it on autopilot, but he declined to speculate as to who might have done so and why. “If the autopilot is operational, it’s because it has been switched on,” Mr. Dolan said.

Based on recent analysis of data from electronic “handshakes” between the plane and a satellite operated by the company Inmarsat, the Boeing 777-200 appears to have followed a straight track to the south, Mr. Dolan added.

Warren Truss, the deputy prime minister of Australia and also the minister for infrastructure and regional development, said at the news conference that Australia planned to hire a contractor to scour a rectangular area of ocean floor covering 23,000 square miles. Up to three deep-sea submersibles will be used for the yearlong endeavor, starting in August.

By comparison, a fruitless search of ocean floor 500 miles farther to the northeast by a United States Navy contractor in late April and May, after the detection of acoustic pings initially believed to have been from the aircraft’s so-called black boxes, covered only 332 square miles.

The new search area runs 400 miles along the “seventh arc” of possible locations for the aircraft, based on the seventh and last electronic handshake that it had with the satellite. The area has a width of only 58 miles on the assumption that the plane was on autopilot and quickly stalled and crashed when it ran out of fuel.

Aircraft and ships searched the new search area for floating debris on the 21st through 26th days after the plane disappeared, and found nothing.

Tim Farrar, a satellite communications consultant in Menlo Park, Calif., one of a group of satellite experts who have been independently analyzing clues to Flight 370’s disappearance, said in a telephone interview that assuming that the plane was on autopilot simplified the search and reduced the range of places along the seventh arc where the plane might have come to rest.

But he cautioned that the electronic handshakes could also be compatible with stable flight controlled by a human pilot. He questioned whether the report or Australian officials had enough information to justify their assumption that the plane was being controlled entirely by autopilot.

Mr. Farrar said that the new search might not find the plane either. “It wouldn’t surprise me if there’s a reasonable probability it’s outside the defined search area,” he said. “It’s going to be a difficult search.”

Correction:July 9, 2014

Because of an editing error, an article on June 27 about a report by Australian officials on the disappearance of Malaysia Airlines Flight 370 over the Indian Ocean in March misstated the conclusions of the report. It said the most likely scenario was that the plane’s crew had become incapacitated and listed as a possible cause hypoxia; it did not conclude that hypoxia was the cause.

A version of this article appears in print on , Section A, Page 4 of the New York edition with the headline: Pressure Loss Is Explored in Vanishing of Jetliner. Order Reprints | Today’s Paper | Subscribe